“Climate change has increased concern over possible large and rapid changes in the physical climate system, which includes the Earth’s atmosphere, land surfaces, and oceans.  Some of these changes could occur within a few decades or even years, leaving little time for society and ecosystems to adapt.  A new report from the National Research Council extends this idea of abrupt climate change, stating that even steady, gradual change in the physical climate system can have abrupt impacts elsewhere — in human infrastructure and ecosystems for example — if critical thresholds are crossed.  The report calls for the development of an early warning system that could help society better anticipate sudden changes and emerging impacts. “Research has helped us begin to distinguish more imminent threats from those that are less likely to happen this century,” said James W.C. White, professor of geological sciences at the University of Colorado, Boulder, and chair of the committee that wrote the report.  “Evaluating climate changes and impacts in terms of their potential magnitude and the likelihood they will occur will help policymakers and communities make informed decisions about how to prepare for or adapt to them.” Abrupt climate changes and impacts already under way are of immediate concern, the report says.  These include the disappearance of late-summer Arctic sea ice and increases in extinction rates of marine and terrestrial species. Other scenarios, such as the destabilization of the west Antarctic ice sheet, have potentially major consequences, but the probability of these changes occurring within the next century is not well-understood, highlighting the need for more research. In some cases, scientific understanding has progressed enough to determine whether certain high-impact climate changes are likely to happen within the next century.  The report notes that a shutdown in the Atlantic Ocean circulation patterns or a rapid release of methane from high-latitude permafrost or undersea ice are now known to be unlikely this century, although these potential abrupt changes are still worrisome over longer time horizons.”